We could rewrite it as 'The Secret of the Author'. This title is more general and can lead to various interpretations, such as the author's hidden life, their writing secrets, or some mystery related to their works.
Another option could be 'The Double - Secret of an Author'. This keeps the idea of there being some sort of mystery or secrets related to the author without any inappropriate connotations. It could imply two main secrets, perhaps one related to their creative process and another to their personal inspiration.